The invention relates to a method for operating a hybrid vehicle and a hybrid vehicle according to the independent claims.
Hybrid vehicles with a multitude of drive units, e.g. a combustion engine and an electric machine coupled to the combustion engine are known in the art. When a hybrid vehicle decelerates, kinetic energy can be recovered by operating the electric machine as a generator and storing the recovered energy in a storage device, e.g. a battery, a super capacitor or the like. The electric machine can be used for a stop-start operation mode of the vehicle. U.S. Pat. No. 7,131,510 B2 discloses a stop-start operation mode of a hybrid vehicle. During a stop stage, the internal combustion engine is disconnected from the electric machine as well as from the transmission system and switched off. In a subsequent start stage, the engine is automatically restarted by the electric machine, which is now operated as a motor, and reconnected to the transmission system. The transmission system is shifted to a predetermined gear during the start stage.
It is desirable to provide a method for operating a hybrid vehicle which allows for improving the recovery of kinetic energy. It is also desirable to provide a hybrid vehicle providing an efficient energy recovery.
A method for operating a hybrid vehicle is proposed, said vehicle comprising at least a first drive unit and a second drive unit coupled to the first drive unit, wherein the second driving unit can recover kinetic energy of the hybrid vehicle, and a transmission unit which can be coupled to the first and/or second drive unit for driving a drive axle.
It is proposed to change the transmission ratio of the transmission unit depending on a desired amount of energy recovered by the second drive unit during regenerative braking or a predetermined time period before the anticipated start of regenerative braking. While braking, the second drive unit is driven in a regenerative braking mode and transforms the vehicle's kinetic energy into a storable energy form which can be used for starting or boosting the combustion engine, e.g. into electric energy stored in a battery or in a super capacitor or e.g. into kinetic energy of a flywheel.
Particularly, the first drive unit can be a combustion engine and second drive unit can be an electric machine which can be operated in a motor mode for driving the drive axle and/or the engine and a generator mode in which kinetic energy can be stored as electric energy when the drive axle drives the electric machine. Preferably the transmission unit can be an automated manual transmission or an automatic transmission.
The higher the rotational speed of the electric machine in the generator mode, the higher is the amount of energy recovered by the electric machine. By operating the electric machine at as high a rotational speed as possible, the energy recovery can be maximized. This can be done by using a low gear of the transmission unit, i.e. a high transmission ratio, during braking. However, the reverse is true for a combustion engine coupled to the transmission unit. A high gear results in lower losses of the engine while driving and a low gear results in higher losses. Advantageously, by using a low gear during braking, the electric machine can run at a higher rotational speed as during driving, when the gear is selected for an optimized operation of the engine. By increasing the transmission ratio, i.e. shifting to a lower gear, the amount of energy recovered during regenerative braking can be increased. By checking if the gear of the transmission unit can be reduced in favour of the rotational speed of the electric machine it can be avoided to start braking from a high gear which is favourable for reducing losses of the engine but reduces the energy recovery during regenerative braking when the electric machine is operated as generator.
Preferably it can be checked if the energy recovered by the second drive unit can be increased during braking by changing a transmission ratio of the transmission unit compared to an actual setting of the transmission unit and to change the transmission ratio if the energy recovered by the second drive unit can be increased. If the transmission ratio is already at its maximum, the ratio cannot be increased further
However, if it is realized that increasing the transmission ratio is not favourable for the energy recovery of the second drive unit, a gearshift is omitted. For instance, if the lowest gear is already set, a change to a lower gear cannot be performed, or if the energy storage is full or nearly full it might not be desired to recover energy. In this case it might even be desirable to reduce the amount of energy recovered by the second drive unit by reducing the transmission ratio and shift to a higher gear. In a preferred alternative, even with a full energy storage a shift to a lower gear can be favourable as the higher rotational speed of an electric machine employed as second drive unit can improve the possibility to use one or both drive units for braking and thus reduce the wear on the disc brakes on the wheels
It is advantageous if the change of the transmission ratio and/or a shut-down of the first drive unit can be initiated prior to braking the vehicle. Shut-down the first drive unit means to cut the fuel supply to the first drive unit and to decouple it from the drive axle, i.e. from the transmission unit and from the second drive unit, preferably by opening a clutch. In one preferred embodiment, the transmission ratio of the transmission unit between the second drive unit and the drive axle can be shifted to a value which is favourable for maximizing the energy recovery by the second drive unit, e.g. by performing a gear shift to a lower gear and thus increasing the rotational speed of an electric machine which is employed as second drive unit. Thus, braking the vehicle can actually start with a favourable transmission ratio, i.e. the gearshift to the desired low gear has been completed or is at least mostly completed and a time loss for energy recovery is minimized. For instance, if an electric machine is used as a second drive unit, no energy at all can be recovered by the electric machine during the time needed for a gearshift. By disengaging the first drive unit a higher transmission ratio can be used at a given speed, because the second drive unit, in case it is an electric machine, can operate at a higher rotational speed than the first drive unit in case the first drive unit is a combustion engine. The gear shifting can be accomplished simpler and faster when it is not necessary to synchronize the speed of the first drive unit. Thus, by an early shut-down of the first drive unit, a disadvantageous shifting of gears during regenerative braking can be avoided.
According to an advantageous method step, the shut-down of the first drive unit can be actively initiated by the driver. A skilled driver usually knows early enough when the vehicle has to be decelerated or stopped. In such cases, the driver can initiate the shutdown by activating an actuator prior to anticipated braking. For instance, the shut-down can be initiated a signal activated by a brake pedal, particularly by tapping the brake pedal before pressing the brake pedal continuously for actually braking the vehicle. In an appropriate drive mode of the vehicle, the tapping of the brake pedal can be identified as a request for initiating the shut-down. A skilled driver can do this well before the instant where an actual braking procedure begins, i.e. the shut-down of the first drive unit can be initiated e.g. at least 500 ms, preferably at least one second prior to braking. This results in energy savings not only by improved energy recovery by the second drive unit but also by an early stop of the first drive unit by cutting the fuel supply.
Additionally or alternatively, the shut-down of the first drive unit can be initiated by a signal activated by a switch operated by the driver. The switch can be an electronic switch or a mechanic switch and can have any form such as a push button, a sliding unit, a lever or the like. The switch can advantageously be arranged at the steering unit such as a steering wheel or a joy stick or the like and can preferably be activated by the driver's hand.
A gear of the transmission unit can be selected for braking. Preferably, the transmission unit can switch to a lower gear when the shut-down of the first drive unit is initiated, thus increasing the time during which an energy recovery is possible while braking the vehicle.
Preferably, the shut-down of the first drive unit can be initiated at least 500 ms prior to anticipated braking.
According to another favourable embodiment of the invention, additionally or alternatively a prediction value can provided for an estimated decision parameter of the hybrid vehicle, e.g. a speed-related parameter or the like. The decision parameter can also be a mathematical function listed variables, e.g. such as mass times speed. Preferably the transmission unit can shift to a lower gear if the prediction value yields the decision parameter beyond a predetermined limit. Such an estimated decision parameter can be for instance a duration of braking, a speed of the vehicle before and after braking, a mass of the vehicle, a slope of the road and the like. If e.g. the estimated duration is long or the change of speed is large, or e.g. the slope of the road is large, a shut-down of the first drive unit as well as a gearshift can be initialized early. It is also possible to combine such parameters for determining if a gearshift is desirable or not. This can be done automatically, without interference of the driver or in combination with a request for shutdown emitted by the driver prior to braking the vehicle, as described above.
Preferably, the transmission unit can shift to a higher transmission ratio, e.g. a lower gear, if the prediction value yields a parameter beyond a predetermined limit. By way of example, the predetermined limit can be a minimum time span. If braking is estimated to be longer than this time span, then it can be decided that changing the gear is desirable. In this case the vehicle will be decelerated to a lower velocity than it was driving before and/or the vehicle is moving downhill thus providing a large amount of kinetic energy for a considerable time which can be recovered and stored. If it can be foreseen that the vehicle will be braking for a long time or the speed change will be large, the gearshift can be done in an optimized way, preferably as early as possible. When the vehicle actually brakes the second drive unit can take full advantage of the braking phase and recover energy. In case a speed change is considered, the predetermined time span can be a minimum speed change. If the speed change is estimated to be larger, then a gearshift can be assumed to be desirable. If the road is inclined or the mass of the vehicle is large, the speed change as well as the duration of braking can be influenced by these parameters.
Particularly, the prediction value can be based on actual position data of the hybrid vehicle, e.g. a GPS system. For instance, if a road crossing is recognized or a parking area or an area with reduced speed or the actual road where the vehicle is driving on is declined, a braking process will most probably hold on for a longer time. The transmission ratio of the transmission unit can be set to a state which is favourable for energy recovery by the second drive unit.
The prediction value can alternatively or additionally be extracted from driving statistics of the hybrid vehicle. If a vehicle is frequently driving the same route and/or the vehicle is driven frequently by the same driver with a particular driving style, e.g. a sportive or a comfort style or braking early or braking late, the prediction value can be adjusted to the particular individual style.
According to another aspect of the invention, a hybrid vehicle is proposed comprising at least a first drive unit and a second drive unit coupled to the first drive unit, wherein the second driving unit recovers kinetic energy during braking the hybrid vehicle, a transmission unit which can be coupled to the first and/or second drive unit and a drive axle. It is proposed to provide a unit for changing the transmission ratio of the transmission unit depending on a desired amount of energy recovered by the second drive unit during regenerative braking. Preferably the first drive unit can be a combustion engine and the second drive unit can be an electric machine. If it is desired to maximize the energy recovery of the electric machine the transmission ratio can be increased. If it is desired to decrease the amount of recovered energy the transmission ratio can be decreased of maintained to the actual value.
Preferably, an actuator is provided for initiating the change of the transmission ratio and/or a shut-down of the first drive unit prior to braking the vehicle. If the amount of recovered energy during regenerative braking should be increased, it is favourable to perform the gearshift as early as possible in order to maximize the time span during which energy can be recovered.
Preferably, the actuator can be coupled to a brake pedal. In a driving mode, tapping on the brake pedal can be interpreted as a request for shut-down of the first drive unit before the driver presses the brake pedal for braking. Alternatively or additionally, the actuator is coupled to a push button accessible for a driver.
The transmission unit can favourably be arranged in the drive train between the second drive unit and driven wheels. The transmission unit can preferably be an automated manual transmission or an automatic transmission. The control algorithm for the transmission unit affects the amount of energy that is recovered by the second drive unit, e.g. by influencing the rotational speed of the second drive unit by providing a selected gear.
Preferably, a clutch can be provided between the first drive unit and the drive train. When a request for shut-down is emitted, the first drive unit can be shut off and decoupled from the drive train early thus saving energy by saving fuel.
Advantageously, a prediction unit can be provided for providing a prediction value of an estimated decision parameter, such as e.g. duration of braking, speed change and the like.
The prediction unit can be coupled to a position detecting system allowing for an estimation of a decision parameter of the vehicle, for instance a duration of a braking phase of the vehicle. Favourably, the prediction unit can be coupled to a data source providing driving statistics. Thus an estimation of the decision parameter can be adopted to an individual driver and/or a particular location of the vehicle.
In the drawings, identical or similar elements are referred to by equal reference numerals. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. Moreover, the drawings are intended to depict only typical embodiments of the invention and therefore should not be considered as limiting the scope of the invention.